International Patent Application No WO 96/31528 (Boden et al.) describes novel rationally designed peptides which self-assemble in one dimension to form beta sheet tape-like polymers. The tapes above a critical peptide concentration (typically above 0.3% v/v peptide) become physically entangled and gel their solutions in organic solvents or in water. The monomeric or single peptide gels possess the specific property of being programmable to switch from the gel state to a fluid or stiffer gel state in response to external chemical or physical triggers. The self-assembly of peptides into beta tape aggregates follows a hierarchical system, as the concentration of peptide increases they will begin to form beta tapes, as the concentration of peptide increases further two beta tapes will interact with each other to form a ribbon, as the concentration of peptide increases yet further ribbons will interact with each other to form fibrils and finally if the concentration increases high enough fibrils can interact to form fibres.
It has recently been found that the tapes having chemically distinct opposing surfaces can give rise to an hierarchy of other self-assembled, supramolecular structures as a function of increasing peptide concentration: ribbons (two stacked tapes), fibrils (many ribbons stacked together) and fibres (entwined fibrils). All these beta-sheet polymers appear twisted because of the peptide chirality. A theoretical model has been developed which rationalizes this self-assembly process of beta-sheet forming peptides using a set of energetic parameters Ej. The magnitudes of Ej define the peptide concentration ranges over which each type of polymer will be stable.
Complementary peptide gels are a special case of peptide gels. The main differences between single peptide gels and complementary peptide gels are that in single peptide gels, gelation can be triggered by specific environmental conditions typically specific pH and/or salinity. This property can create a problem in the case of usage of peptide gels in medical applications, i.e. the peptide fluid solution eg in pure water, hits the physiological solution and immediately transforms into a gel, which can act like a gel plug, preventing further diffusion of the peptide solution to fill a large cavity or to form an interpenetrated network inside another porous matrix for example a decellularized tissue matrix. In the case of the complementary peptide gels, it is possible to overcome this problem by administering first peptide A which is in a low viscosity fluid monomeric state, this is then followed by administering the complementary peptide B which is also in a low viscosity fluid monomeric state. In this case, the formation of the peptide gel network only takes place by the coexistence of A and B in the same volume and their interaction and self-assembly (FIG. 1), rather by the presence of any other chemical or physical conditions of the solution, i.e. pH, salinity or specific counterions e.g., Ca+2. This makes complementary peptide gelation in situ a much more reliable event and much more likely to happen in the whole space that is available rather than only at the entrance point of a cavity or only on the surface of a porous material.
A further difference between single peptide and complementary peptide self-assembly is that the latter typically relies on complementary intermolecular electrostatic interactions. This causes very high affinity between adjacent self-assembling peptides, much higher than it would normally by achieved by single peptide self-assembly. Since the affinity between complementary peptides is expected to be higher than for single peptides, then the critical concentration (c*tape) for tape self-assembly will be expected to be much lower for complementary peptides than for single peptide tapes. The magnitude of c*tape (FIG. 2) relates to how fast or how slow a peptide gel will dissolve out of the injection site in situ. Peptide gels that are required to have as long a lifetime as possible in vivo, must have as low c*tape value as possible. Therefore complementary peptides provide a way to form an injectable gel in situ that will be expected to be much more long lived and therefore acting for much longer in vivo, than their corresponding single peptide gels.
A yet further difference between single peptide tapes and complementary peptide tapes is that the complementary ones provide a lot more surface versatility than single peptide gels because they consist of alternating peptides A and B (FIG. 3). Therefore they provide new opportunities to control distances between functional groups and to introduce new surface functionalities, thus extending the possible bioactive properties of this class of peptide gels.